Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax(SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In, Ga, Al)(S, Se, Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In, Ga, Al)(S, Se, Te)2, is grown over a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the contact layer 13 form a base 20. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a contact layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In, Ga, Al)(S, Se, Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 or CIGS(S) means the whole family of compounds with the Ga/(Ga+In) molar ratio varying from 0 to 1, and the Se/(Se+S) molar ratio varying from 0 to 1.
Cu(In,Ga)(S,Se)2 type compound thin films may be deposited over the selected substrate by various methods such as co-evaporation, sputtering/co-sputtering, ink deposition, electrodeposition, etc., One technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where metallic components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CIS or CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CIS(S) or CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a CIGS(S) or Cu(In,Ga)(S,Se)2 absorber. The precursor layers may be deposited by various methods such as evaporation, sputtering, ink deposition, electrodeposition, etc.
Two-stage process approach may also employ stacked layers comprising Group VIA materials. For example, a CIGS or Cu(In,Ga)Se2 film may be obtained by depositing In—Ga—Se and Cu—Se layers in an In—Ga—Se/Cu—Se stack and reacting them in presence of Se. Similarly, stacks comprising Group VIA materials and metallic components may also be used. Stacks comprising Group VIA materials include, but are not limited to In—Ga—Se/Cu stack, Cu/In/Ga/Se stack, Cu/Se/In/Ga/Se stack, etc. The stacks may be deposited over the substrate using the various methods listed above.
Building Integrated Photovoltaics (BIPV) used in buildings often needs semitransparent solar cells and modules that can be utilized on building facades. These devices, while generating electricity, also let some predetermined amount of the light impinging on the solar module into the building. The semitransparent PV modules may have a transparency in the 10-70% range.
One way of achieving semi-transparency in PV modules is to use a transparent front protective sheet and back protective sheet in the module structure and leave large gaps between the solar cells in the module. The larger the gaps are between the solar cells, the more sun light passes from the front side of the module to the back side. However, this method is not attractive because as the space between cells increases the interconnection wiring or the interconnection ribbons that electrically connect each solar cell with its neighboring cell becomes more and more visible.
Modules made of amorphous silicon can also be made partially transparent by employing thin absorber layers and transparent contacts. In such amorphous silicon applications, the transparency of the solar cell itself can be controlled by the thickness and type of the amorphous silicon. This method cannot be used for CIGS and CdTe type solar cells because these solar cells employ metallic contact layers that are opaque.
One other method used to achieve semi-transparency for crystalline silicon solar cells involves mechanical texturization of the front and rear side of the silicon wafer before the cell is fabricated. In this method, perpendicular grooves made on the front and rear sides of the silicon wafer create holes at their crossing points if the depth of each groove is larger than half of the thickness of the Si wafer. The size of the holes can be controlled by controlling the depth of the grooves, deeper grooves creating larger holes. The holes in silicon solar cells result in a partial optical transparency of the device. A hole size obtained with such methods is typically in the range of 100-200 μm diameter. These devices are extremely fragile because the grooves penetrate into the silicon along substantially the whole length of the solar cell and reduce the mechanical strength of the substrate, which typically have a total thickness in the 200-400 μm range. Therefore, in order to optimize the mechanical strength of the device, the hole size is limited so that the transparency of the device is typically in the range of 15-25% range. The grooves opened in the Si substrate also make solar cell processing difficult. Thin film solar cells have absorber layer thicknesses in the 1-10 μm range. Therefore, the grooving method cannot be used in such devices. Grooves with depths of 0.5-8 microns would yield holes with diameters in the range of only a few microns. Furthermore, fragile nature of thin films would not allow defect free grooving with the required precision.
Therefore there is a need for robust, semi-transparent thin film flexible solar cells that can be handled without concern for breakage.